GB2349700A

GB2349700A - Flowmeter with diaphragm and strain gauges

Info

Publication number: GB2349700A
Application number: GB0006923A
Authority: GB
Inventors: Chris Engstrom; Ake Eriksson
Original assignee: Eletta AB
Current assignee: Eletta AB
Priority date: 1999-05-06
Filing date: 2000-03-23
Publication date: 2000-11-08
Anticipated expiration: 2020-03-23
Also published as: SE9901640L; DE10016032C2; JP2001091318A; DE10016032A1; US6543290B1; ITFI20000104A0; IT1314614B1; FR2793319B1; JP3263691B2; GB2349700B; ITFI20000104A1; SE514106C2; GB0006923D0; SE9901640D0; FR2793319A1

Abstract

The flowmeter, in which the fluid is made to pass tnrough a primary element (20) and in which the pressure on either side of a restrictor (2a) is allowed to act on either side of a diaphragm (11) mounted in a diaphragm housing (3), which diaphragm, in turn, acts on a measuring element (17) provided with strain gauges (27) connected to a measuring bridge (5), the signals from which can be used to indicate the flow, for example on an indicating instrument (7), or otherwise to record changes in the flow or control the flow at a desired value by means of a control system. The element (17) has a finger (22) fixed at its bottom end in a passage (15) at right-angles to a diaphragm spindle (8), the finger being adjustably held in a spindle passage 16 by a screw (23) with a tapered end.

Description

DESCRIPTION FLOWMETER The present invention relates to flowmeters, primarily for fluids.

Devices are known for measuring the flow or pressure of a fluid, in which the fluid passes through a primary element, and in which the pressure on either side of a restrictor in the said primary element acts on either side of a diaphragm mounted in a diaphragm housing, which diaphragm, in turn, acts on a measuring element provided with strain gauges and connected to a measuring bridge which indicates the flow through the primary element. In pressure measurement, the fluid pressure acts on one side of the diaphragm only.

In previously known flowmeters based on the strain gauge principle, such as US2,707,393, which is designed to measure whether and to what degree a flow is pulsating, the movements of the diaphragm are transferred to the body housing the strain gauges by means of a linkage, which gives rise to unwanted hysteresis effects as the flow increases and decreases. Since the strain gauges are in direct contact with the fluid, the results of this measurement are also affected by changes in fluid temperature. This is true, for example, of US 3,444,736, a complicated arrangement in which the diaphragm which actuates the strain gauges is surrounded by silicone oil and is enclosed between two further diaphragms which, in turn, detect the pressure differential in a venturi tube. This device also causes unwanted hysteresis effects and, according to the patent document, affords a flow ratio of only 3: 1. Furthermore, in both of these devices, the electrical connections from the strain gauges, which are immersed in pressurised fluid, must be provided with fully sealed electrical penetrations.

An object of the present invention is to seek to eliminate or minimise the disadvantages associated with previously known devices.

In accordance with the present invention there is provided a device for measuring flow, in which the fluid in question is made to pass through a primary element provided with a restrictor and in which the differential pressure across the said restrictor is allowed to act on a diaphragm which, in turn, acts on a measuring element provided with strain gauges connected to a measuring bridge which indicates the flow through the restrictor, the movements of the diaphragm being transmitted, without play, through a diaphragm spindle to a finger, which is arranged at right angles to the diaphragm spindle and is free to move in a hollow passage in a cylindrical measuring element, and is fixed at the lower end of the hollow passage.

A flowmeter in accordance with the invention can have the advantage of having a simple construction which is entirely free of hysteresis effects as the flow increases and decreases, and which, by virtue of the fact that the strain gauges are located completely separate from the fluid and are mounted symmetrically on a measuring element, provides measured values which are completely independent of temperature changes in the fluid. A further advantage is that the fluid cannot damage the strain gauges through its aggressivity, toxicity, temperature, pressure or abrasive properties.

In a preferred construction, the diaphragm mounted in a diaphragm housing is acted on by the higher and lower pressure present at either side of the primary element, and the diaphragm spindle extends at right angles to the diaphragm proper, one end of the spindle being attached to the centre of the diaphragm and the other being provided with a free-fit hole. One end of the finger is held, without play, by an adjusting screw located centrally in the hole in the free end of the diaphragm spindle. The finger extends downward through a hollow measuring element, to which it is fixed at the lower end. The measuring element, the hollow interior of which is open to the fluid on one side of the diaphragm, is bolted to one side of the diaphragm housing in a liquid-tight manner. Since the diaphragm tends to move towards the side on which the lower pressure acts, it exerts a pulling or pushing action on the diaphragm spindle depending on the side on which the high pressure is allowed to act. The diaphragm spindle, in turn, deflects the measuring element through the finger attached to it. The strain gauges bonded to the outside of the measuring element are connected, in conventional manner, to a Wheatstone bridge, which converts the changes in resistance of the strain gauges into an electrical signal This signal can be used to provide a flow-calibrated reading on an indicating instrument and/or otherwise to record changes in flow or to control the flow at a desired value with the aid of a known control system.

The present invention also affords the possibility of a flow ratio of 10: 1 to 100 : 1 when measuring pressure only, which is significantly higher than that of previously known devices.

The invention will be described hereinafter in further detail by way of exatple, only with reference to the appended figures, of which Fig. 1 is a schematic of a flow measuring device and Fig. 2 is a vertical section through the diaphragm housing of a flowmeter.

Fig. 3 is a side view of the diaphragm housing.

Fig. l shows a pipeline 1 containing a medum whose flow is to be measured and which passes through a restrictor 2a in a primary element 2, on either side of which restrictor the fluid is connected to a diaphragm housing 3 by instrument lines 4. A branch 18 which houses a measuring element (17 in Fig. 2) is bolted to the diaphragm housing, to which a measuring bridge 5, complete with its power supply unit 6, and an indicating instrument 7 showing the flow in suitable unitsare also attached.

Fig. 2 is a vertical section through the flowmeter diaphragm housing 3, between the right-hand section 9 and left-hand section 10 of which a diaphragm 11 is clamped in a liquid-tight manner, using O-ring seals 12, by means of bolts 13. A diaphragm spindle 8, which extends freely through a hollow passage 14 in the left-hand section of the diaphragm housing, is attached, at one end, to the centre of the diaphragm. The other end of the spindle is provided with a hole 16 drille at right angles to the centre line of the diaphragm spindle. The left-hand section of the diaphragm housing is also provided with a hollow passage 15 whose centre coincides with that of hole 16. A hollow measuring element 17 is arranged centrally around the hollow passage 15 and is secured in position in a liquid-tight manner by branch 18 by means of bolts (19 in Fig. 3) and O-ring seal 20. A finger 22 attached to the lower end 21 of the measuring element is free to move in the hollow interior of the measuring element, through the hollow passage 15 and centrally through the hole 16 in the diaphragm spindle. The free end of the diaphragm spindle is provided with a central, threaded hole fitted with an adjusting screw 23, the tapered end of which joins the diaphragm spindle 8 and finger 22, without play, at a hole 24 drilled through the finger in line with the centre line of the diaphragm spindle. The measuring element 17 is aligned relative to the left-hand section of the diaphragm housing by means of a guide pin 25 to ensure that the hole 24 in the finger is aligned with the centre line of the diaphragm housing when installing the measuring element. Strain gauges 27 are fitted in pairs on an outer, waisted mid-section 26 of the measuring element and are connected electrically to the measuring bridge 5.

Furthermore, the right-hand and left-hand sections respectively of the diaphragm housing are provided with threaded holes 28 for connecting the instrument lines 4 from the restrictor primary element 2.

Fig. 3 is a side view of the left-hand section 10 of the diaphragm housing showing the branch 18 secured by the bolts 19. The right-hand and left-hand sections of the diaphragm housing are joined by the bolts 13. The measuring bridge is designated 5.

Since only the differential pressure is measured, the present invention is completely independent of the static pressure of the fluid and of any changes in pressure, and is not limited to the embodiment described, but is also valid for variants of the said embodiment. The invention may also be used to measure both liquid and gas flows. In addition, the invention may be used to measure the static pressure in a pipeline by connecting one of the instrument lines 4 to the pipeline in which the pressure is to be measured. The invention may further be used to measure the level in a vessel when the specific weight of the liquid is known and the liquid pressure is measured at the bottom of the vessel. If the vessel is closed and under higher pressure than ambient, the second instrument line 4 of the invention should be connected to the gas phase (that is, to the top of the vessel).

Claims

1. A device for measuring flow, in which the fluid in question is made to pass through a primary element provided with a restrictor and in which the differential pressure across the said restrictor is allowed to act on a diaphragm which, in turn, acts on a measuring element provided with strain gauges connected to a measuring bridge which indicates the flow through the restrictor, the movements of the diaphragm being transmitted, without play, through a diaphragm spindle to a finger, which is arranged at right angles to the diaphragm spindle and is free to move in a hollow passage in a cylindrical measuring element, and is fixed at the lower end of the hollow passage.

2. A device as claimed in claim 1, wherein the finger is free to move in a hole formed in the diaphragm spindle and at right angles to the spindle's centre line, and is held in tension with the diaphragm by means of an adjusting screw having a tapered tip, the screw being threaded centrally into the end of the diaphragm spindle with its tip seated against a hole formed in the finger and its centre aligned with the centre line of the diaphragm spindle.

3. A device as claimed in claim 1 or 2, wherein the cylindrical measuring element is provided externally with a waisted mid-section on which strain gauges are mounted in pairs.

4. A device for measuring fluid flow, substantially as hereinbefore described, with reference to and as illustrated in the accompanying drawings.